Superphosphate, manufactured by treating phosphate rock with sulphuric acid, has been a mainstay of New Zealand farming. Phosphate is also available as partially acidulated phosphate and as untreated phosphate rock. This brief exploration of the phosphate fertiliser raises questions about its continued use and contrasts Māori and Pākehā perspectives.
The story of the terra preta soils in the Amazon is well known. Not so well known is how pre-European Māori used charcoal, gravel and sand to increase the quality of soils in the Waimea plains and other soils around New Zealand. In their 1923 paper in The Journal of Polynesian Society, authors T Rigg and J.A. Bruce were clearly impressed by soil quality, and the soils were notably still of superior quality when they tested them possibly 100 years after Māori lost access to them. The gravel and sand applied improved the soil texture and Bruce and Rigg attributed the very high levels of phosphate to the burning of scrub.
The source of the enrichment was apparently wood ashes, since the soil is black, owing to the presence of much charcoal. Wood, scrub, or other vegetable matter must have been brought on to the land and there burnt. Tea-tree (manuka) is suggested as the form of vegetable matter which was employed for this purpose. The ash of tea-tree is rich in phosphates, potash and lime. (Rigg and Bruce, p. 9[i]).
A recently published book by Ewen Campbell identified how a Māori associate reported his father’s reaction to the use of superphosphate on the neighbouring farm.
I was with Bob Maru, an ex-World War II Māori Battalion member in Gisborne. In his later years Bob was attempting to reinvigorate the East Coast utilising Māori land. After I had spoken to a group of his colleagues, he came to me with a grin on his face and stated that he had finally worked out what his father had spoken to him about when he was seven years old. He was about eighty at that stage, so he had waited a fair while for the answer. He recounted to me that he and his dad were in the garden while they were applying superphosphate on the neighbouring property. His father commented to Bob that “those white fellas are stealing our mauri”. Mauri is the Maōri word for “life force”. I had informed the group of how acid fertiliser binds silica to aluminium and therefore the silica loses its electrical production capability and in doing so changes the nutrient delivery system from electrical to soluble so, in effect, as Bob’s father observed, the life force is taken (Campbell, p. 97[ii]).
From the clues in the text the date of Bob’s conversation with his father would be between 1900 and 1920. Superphosphate production began in Aotearoa in 1882[iv]. After World War Two, returning pilots found work topdressing superphosphate, increasing the spread of hill country farming.
Aotearoa has accessed its phosphate from Nauru and more recently from Western Sahara with detrimental consequences. About 90% of Nauru is now covered in jagged and exposed petrified coral and runoff into the Ocean has impacted on marine life[v].
Now, fertiliser companies import about $30 million of phosphate from West Sahara every year. This country was taken over by neighbouring Morocco in 1975 despite protests from the United Nations. The Moroccans dominate the control of phosphate. In 2019 representatives of the New Zealand Government officials met with a representative of the Western Sahara independence movement and agreed that we should be sourcing our phosphate elsewhere[vi]. Kim Hill’s interview of Matthew Galloway provides further detail about Western Sahara’s “blood” phosphate.
In interviews with a kaumātua [Māori elder], Benjamin Pittman related how his grandfather, Okeroa, refused to have artificial fertilisers on his farm.
How does this imported fertiliser perform? Results from soil tests on a local organic farm revealed Olsen P levels of 23, on the low end of the medium range. This would normally prompt recommendations for the addition of phosphates. But herbage tests revealed high levels of phosphorus (0.49%). This indicates an active micro-biome on the organic farm that effectively mines phosphate from soil minerals and makes it available to plants.
Beside perpetuating injustices in countries of origin, phosphate fertilisers do harm here too. They contain cadmium and fluoride. The Ministry of Primary Industries claim that cadmium is within World Health Organisation guidelines. Cadmium is a heavy metal that can harm human and ecosystem health[vii],[viii]. Phosphate fixes strongly to the clay fraction of soil. In Aotearoa “over two hundred million tonnes of sediment are lost … into the Ocean every year”.[ix] This is the main pathway for phosphates to pollute our waterways.
When I was taught soil science in the late 1970s, I learned that when minerals are taken off the land in produce, they have to be replaced. Forty years on, Australian soil scientist, Christine Jones, told me that what is exported is negligible compared to the store of minerals available in the soil. The micro-biome effectively mines these minerals originating from parent material in the age-old process of pedogenesis. In addition to the electrical dimension of mauri referred to above, we can include the life force of the micro-biome.
We can excuse those who applied the fertiliser in the past, but now we know more about the soil biome and the work it does in supplying plants with minerals, surely the costs outweigh the benefits. There are alternatives to highly soluble fertilisers. We have a lot to learn and a lot to unlearn. If the voice of Māori had been better respected, perhaps we could have reduced the negative impacts of using soluble fertilisers?
This post is adapted from Peter’s soon-to-be-published book “How plants cool and heal the climate”. This in turn was adaped from “Whakaora ngā whenua whāma: Utilising mātauranga Māori and western science to protect and restore the soil on rural farms in Te Tai Tokerau“
[i] T Rigg and J.A. Bruce, ‘Journal of the Polynesian Society: The Maori Gravel Soil Of Waimea West, Nelson, New Zealand’, The Journal of Polynesian Society 32, no. 126 (1923): 85–93.
[ii] Ewan Campbell, An Ecofarmer’s Discovery: How the Soil Really Works, 2nd ed. (Aotearoa: Ecofarm Aotearoa, 2020), http://ecofarmaotearoa.com/wp-content/uploads/2020/07/An-Ecofarmers-Discovery-edition-2.pdf.
[iii] ‘Autumn Top Dressing with Superphosphate’, The Te Aroha News, 10 May 1924.
[v] Amber Pariona, ‘How Has Phosphate Mining in Nauru Led to an Environmental Catastrophe?’, WorldAtlas, 2017, https://www.worldatlas.com/articles/how-phosphate-mining-in-nauru-has-led-to-an-environmental-catastrophe.html.
[vi] RNZ, ‘“Blood Phosphate” Imports from Western Sahara May Prompt Illegal Port Strikes – Union’, RNZ, 14 December 2019, https://www.rnz.co.nz/news/national/405491/blood-phosphate-imports-from-western-sahara-may-prompt-illegal-port-strikes-union.
[vii] Ministry for Primary Industries., ‘Cadmium’, 27 March 2019, https://www.mpi.govt.nz/protection-and-response/environment-and-natural-resources/land-and-soil/cadmium/.
[viii] Nick Kim, ‘Cadmium Accumulation in Waikato Soils’, Waikato Regional Council, 2005, http://www.waikatoregion.govt.nz/services/publications/technical-reports/tr/tr200551/.
[ix] John Dymond, ‘Soil Erosion in New Zealand Is Sinking Carbon (C) at 3 Million Tonnes per Year’, Manaaki Whenua – Landcare Research, September 2010, http://www.landcareresearch.co.nz/publications/newsletters/soil/issue-19/soil-erosion.